Rapid advances in biotechnology have led to the discovery of numerous protein and peptide therapeutics, many of which have recently reached the marketplace or are currently under regulatory review by the United States Food and Drug Administration. Unlike traditional small-molecule drugs, however, proteins and peptides generally cannot be administered orally; injection or infusion is most often required. Further, because of their fragility and short in vivo half-lives, encapsulation of proteins in biodegradable polymeric devices, from which the drug can be delivered, locally or systemically, for a prolonged period of time, has been a promising and intensely studied solution to these problems.
Biodegradable microparticles containing a variety of polymers have been the most studied delivery vehicle due to relatively simple fabrication and facile administration to a variety of locations in vivo through a syringe needle. One particularly preferred microparticle material is chitosan, a hydrophilic biodegradable natural polymer that has been used for drug delivery systems in recent years. To manufacture microparticles of chitosan for this type of application, various methods have been employed, including spray drying and classic emulsions, both at the bench and industrial scales. However, neither technique has yielded uniform microparticles and microcapsules (to be collectively referred to as microparticles) with precisely controlled size and size distribution. In fact, standard deviations equal to 25-50% of the mean diameter are not uncommon.
Control of particle size and size distribution has several important implications for controlled-release drug delivery. For example, there typically is an ideal particle size that provides a desired release rate and route of administration. Microparticles that are “too small” exhibit poor encapsulation efficiency, may migrate from the site of injection, and may exhibit undesirably rapid release of the drug. Particles that are “too large” may not easily pass through a syringe needle. Thus, the typical polydisperse particles generated by conventional fabrication techniques must be filtered or sieved to isolate particles within the desired size range, and the particles outside that range are wasted.
Moreover, with traditional technologies for spraying microdroplets from nozzle-type devices, the minimum particle size typically obtainable is limited by the size of the nozzle opening. Usually, it is not possible to make drops smaller than the nozzle opening; typically, droplet diameters are 1-4 times the diameter of the nozzle. This presents several difficulties as the desired particle size decreases. One problem is that fabrication of the nozzles themselves becomes more difficult as size decreases.
A second limitation stems from the pressure needed to pump fluids through small nozzles. The pressure required scales with R−4, wherein R is the radius of the nozzle. Thus, pumping virtually any liquid through a nozzle of 5-μm diameter would require special equipment, if it could be done at all. Also, some compounds to be encapsulated, such as plasmid DNA, may be damaged by shear forces. In general, the damage is approximately inversely proportional to the diameter of the orifice. Thus, decreasing the nozzle diameter from 100 to 5 μm would increase the damage done to any encapsulated compound by a factor of 20.
Published U.S. Patent Application No. 2002/0,054,912 (published May 9, 2002, entitled “Microcapsules”, to Kim et al.) hereby incorporated by reference, teaches a process wherein micro- and nano-sized particles, preferably spherical, are produced by pumping material, usually a polymer dissolved in an organic solvent, through a small orifice and then shaking the liquid with an acoustic type-wave, where the velocity of the fluid is increased beyond the velocity produced by pressure behind the liquid. In this process, the nozzle diameter may be larger than the particles produced. For example, 5-μm droplets can be prepared from a much larger nozzle, such as a nozzle of 100 μm diameter. The droplets are collected in a solution, and the presence of a surfactant prevents the droplets from sticking together before the evaporation of the organic solvent leads to the hardening of the droplets into microparticles.
The pressures needed to form very small particles are reduced to ranges easily obtained with commercial high-pressure pumps such as those commonly supplied with high-pressure liquid chromatography systems. Furthermore, the shear forces are greatly reduced for a given particle size, and the difficulties encountered with very small diameter nozzles are also eliminated. Aspects of the invention are described in “Fabrication of PLG microspheres with precisely controlled and monodisperse size distributions” J. Controlled Release 73(1):59-74 (May 18, 2001).
The vibration or shaking can be achieved by, for example, a piezoelectric transducer driven by a wave generator, and breaks the stream into a train of uniform droplets. Droplet size is determined by the orifice diameter, the solution flow rate, the vibration frequency and amplitude. Thus, by varying these four parameters droplet size can be controlled.
The velocity of the fluid is increased beyond the velocity produced by the pressure behind the liquid by employing an additional downward force that will ‘pull’ the liquid jet through the orifice, reducing the jet size below the diameter of the orifice. One example is an electrohydrodynamic technique in which electrical forces act to reduce the diameter of the liquid jet and the resulting droplets. The electrohydrodynamic technique is activated through injection of charge of desired polarity into the liquid by applying a high voltage either to the nozzle or directly into the liquid, for example, with a battery, or with a transformer and a rectifier to convert standard current. Outwardly directed electrical tension forces result at the charged liquid meniscus of the nozzle opening, enabling a smaller drop to fall from the nozzle (the “drip mode”). The reason for this reduction in drop size is believed to be that there are two forces present, gravitational and electrical, that are working together to pull the liquid off of the nozzle, while surface tension forces hold the liquid at the nozzle. As the amount of charge injected increases, the electrical tension forces accordingly increase, eventually dominating the gravitational and surface-tension forces and reducing the drop size. Further increase in charge injection beyond a certain threshold value results in very powerful electrical tension forces that literally pull the liquid out of the nozzle to form a thin charged liquid jet, which in turn breaks up into fairly uniform droplets (known as the “jet mode”). Jet mode changes from single-jet to multi-jet mode as charge injection is further increased.
Another example of an additional downward force employed is a separate liquid stream (typically immiscible) through the orifice, adjacent and parallel to the particle-forming liquid, at a velocity greater than the particle-forming liquid. The particle-forming liquid is pulled along by the drag forces at the liquid/liquid interface. The particle-forming jet is reduced in diameter by a factor that is proportional to the difference in linear velocities of the two streams.